Surface acoustic wave device and method of adjusting LC component of surface acoustic wave device

ABSTRACT

A surface acoustic wave device comprises a piezoelectric substrate ( 1 ), at least one inter-digital transducers (IDT)( 2 ) provided on the piezoelectric substrate, at least one elongated electrode pad ( 4 ) electrically connected to the IDT, and at least one stud bump ( 5 ) disposed on the electrode pad such that an LC component of the surface acoustic wave device has a predetermined value.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a surface acoustic wave device and amethod of adjusting an LC component thereof, especially a surfaceacoustic wave device mounted on a base substrate by a stud bump and amethod of adjusting an LC component thereof.

2. Description of the Related Art

A conventional surface acoustic wave (SAW) device is disclosed in, forexample, Japanese Patent Application Kokai Number 2000-114916 (pages 4-5and FIGS. 1-2). In the conventional SWA device, an SAW chip containingan inter-digital transducer (IDT) is mounted on a base substrate by theflip chip mounting method. Each of electrode pads has a top end which isprovided with a bump and disposed in the center of the SAW chip. Sincethe bump is disposed in the vicinity of the center of the SAW chip, thestress produced by the difference in the coefficient of thermalexpansion between the SAW chip and the base substrate is reduced, thuspreventing wiring defective caused by heat.

In an SAW device, the characteristics of individual device are often notuniform if the thickness and shape of the IDT and the electrode pad areuneven. When the SAW device is used under high operational frequency,the influence of the uneven thickness and shape is large. Especially, inan SAW filter, such as an SAW duplexer, it is required that an LCcomponent of the individual SAW device is adjusted to achieve apredetermined resonant frequency. However, in the SAW device disclosedin Japanese Patent Application Number 2000-114916, the position of thebump is limited to a specific area of the device so that the degree offreedom of the bump position is not large and it is difficult to adjustthe LC component of the individual SAW device and also the LC componentat a specific position in the SAW device.

SUMMARY OF THE INVENTION

A surface acoustic wave device according to the present inventioncomprises a piezoelectric substrate, at least one IDTs, at least oneelectrode pad, and at least one stud bump. IDT is provided on thepiezoelectric substrate and electrically connected to the electrode pad.The stud bump is disposed on the electrode pad such that the LCcomponent of the surface acoustic wave device has a predetermined value.

The IDT is an abbreviation for inter-digital transducer and a kind ofconverter which converts an electrical signal to a surface acoustic waveand returns the surface acoustic wave to the electrical signal incollaboration with the piezoelectric substrate, thus transmitting theelectrical signal.

According to the present invention, the stud bump is disposed on theelectrode pad, adjusting the position of the disposition such that theLC component of the surface acoustic wave device has a predeterminedvalue. In case of a plurality of IDTs provided in the device, the LCcomponent can be adjusted for an individual IDT by adjusting thedisposition position on the electrode-pad connected to the individualIDT. In case that the SAW device is an SAW filter, such as an SAWduplexer, even if the resonant frequency of the SAW filter is fluctuatedby the manufacturing variations, the resonant frequency can be correctedby adjusting the LC component.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view of an SAW chip according to the first embodiment ofthe present invention.

FIG. 2 is a top view of an SAW chip according to the second embodimentof the present invention.

FIG. 3 is a top view of an SAW chip according to the third embodiment ofthe present invention.

FIG. 4 is an illustration of the SAW chip showing an example Of mountingthe SAW onto a base substrate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS (1) First Embodiment

(1-1) Construction

FIG. 1 is a top view of a surface acoustic wave (SAW) chip 100 accordingto the first embodiment of the present invention. Here, an SAW chip foran SAW duplexer, which is applicable for an antenna stage of mobilecommunication unit of a portable telephone, is exemplified.

The SAW chip 100 comprises IDTs 2 a-2 e, reflectors 3 a-ah, andelectrode pads 4 a-4 d, formed on a piezoelectric substrate 1. Studbumps 5 a-5 k, which become connection portions when mounting the SAWchip 100 onto a base substrate, are provided on the electrode pads 4 a-4d. The piezoelectric substrate 1 is made of a piezoelectric material,such as LiTaO₃, LiNbO₃, and quartz crystal. The IDTs 2 a-2 e are made ofAl or an Al alloy containing a small amount of Cu. The IDTs 2 a-2 e areelectrodes having a shape of a comb, wherein the teeth of the comb areopposed to each other. The IDTs 2 a-2 e convert a high frequencyelectrical signal to a surface acoustic wave and re-convert and outputthe surface acoustic wave to the high frequency electrical signal. Asshown in FIG. 1, the reflectors 3 a-3 h are disposed at sides of theIDTs 2 a-2 e. For example, the reflectors 3 a and 3 b make surfaceacoustic wave generated from the IDTs 2 a and 2 b reflect and travel inmultiple by resilient and electrical disturbing effect, thus generatingstanding waves by overlapping permeating waves and reflected waves. Thereflectors 3 c-3 h are similar to the reflectors 3 a and 3 b. Theelectrode pad 4 a is electrically connected to the input portion of theIDTs 2 and 2 d, and the electrode pads 4 b, 4 c, and 4 d areelectrically connected to the output portions of the IDTs 2 e, 2 c, and2 b, respectively. The electrode pad 4 a has an elongated top viewhaving a substantially reverse “C” and the stud bumps 4 b, 4 c, and 4 dare disposed at arbitrary positions thereof. The electrode pads 4 b, 4c, and 4 d have also elongated forms, respectively, and the stud bumps 5h-5 k are disposed at arbitrary positions thereof. The electrode pad 4 bis extended in a widthwise direction thereof too so that the position ofthe stud bump 5 h can be selected from wide range of position in thewidthwise direction. The IDTs 2 a-2 e, the reflectors 3 e-3 h, and theelectrode pads 4 a-4 d are formed by the ordinary method 6 f forming ametal film, which includes the processes of resist coating, exposure,and etching performed after metal sputtering. That is, the electrodepads 4 a-4 d can be formed through the processes of forming an electrodefilm, photolithography, and etching like wire bonding pads. Au balls areformed on the electrode pads 4 a-4 d by exclusive bonder to prove thestud bumps 5 a-5 k. That is, the stud bumps 5 a-5 k are crimped on theelectrode pads 4 a-4 d by using ultrasonic waves or heat after decidingthe positions of the stud bumps 5 a-5 k. The stud bumps 5 a-5 k may bemade of AuPd, Cu, and solder instead of Au.

In FIG. 4, electrode pads 4 a′-4 d′ corresponding to the electrode pads4 a-4 d are formed on a base substrate 20. When mounting the SAW chip100 onto the base substrate 20, conductive paste containing solder iscoated on the stud bumps 5 (5 a, 5 g, 5 h, 5I, and 5 k) or the electrodepads 4 a′-4 d′ and the SAW chip 100 is disposed on the base substrate 20such that the electrode pads 4 a-4 d mate with the electrode pads 4 a′-4d′, respectively. At this point, as shown by arrows in FIG. 4, the studbumps 5 are mated with the stud bumps 5′ (5 a′, 5 g′, 5 h′, 5 i′, and 5k′) in the electrode pads 4 a′-44 d′. For simple explanation, only thestud bumps 5 a, 5 g, 5 h, 5 i, and 5 k are described, however, the otherstud bumps are made in the similar way. When the conductive pastebecomes solid, the SAW chip 100 and the base substrate 20 are fixed sothat the electrode pads 4 are electrically connected to the electrodepads 4′ through the stud bumps 5. Even when the number of connectionpoints of the stud bumps 5 on the electrode pads 4 is increased ordecreased or the connection position of the stud bumps 5 on theelectrode pads 4 is changed, the SAW chip 100 can be mounted onto thebase substrate 20.

In the above description, the reflectors 3 are provided at the sides ofthe IDTs 2, however, the reflectors 3 may be omitted if the internalreflection of the IDTs 2 can be used.

(1-2) Effects

If there is a wire-bonding between pads of an SAW chip and a package,the adjustment of an L component can be done by bonding one of the padsof the SAW chip to a specific pad or a plurality of pads of the package,or by disposing the pads such that the distance between the pads of theSAW chip and the package is made large. However, in case of an SAWdevice of flip-type mounting, wherein no wire-bonding is conducted, ithas been extremely difficult to adjust the L component because thepositions and the number of the stud bumps are fixed. That is, even ifit is found that desired L component was not obtained by manufacturingvariations after designing the layout of IDTs, electrode pads, etc., theL component can not be adjusted because of the fixed positions andnumber of the stud bumps. Consequently, the pattern layout must bechanged.

In the SAW chip 100 according to the present invention, since theelectrode pad 4 is elongated, the number of the stud bumps 5 provided inthe electrode pad 4 can be increased or decreased and the stud bumps 5are disposed at arbitrary positions so that the LC component of the SAWchip 100 can be adjusted. If the LC component is adjusted for each ofthe electrode pads 4 a-4 d, the LC component at a specific position ofthe SAW chip 100 can be adjusted. The LC component can be also adjustedby changing the length of an open stub, which is provided in front ofeach of the stud bumps 5 in the elongated electrode pad 4, by changingthe positioning of the stud bumps 5. Especially, in an SAW filter, suchas an SAW duplexer, the LC component can be adjusted to adjust theresonant frequency by changing the number and positions of the studbumps 5. The characteristics of the finished product, wherein the SAWchip 100 is mounted on the base substrate 20 and packaged, are confirmedin the ordinary test after packaging to determine the optimum positionsand the number of the stud bumps 5 for the mass-production.

An example of a plurality of the stud bumps 5 a-5 k is described in theabove, however, the LC component may be adjusted by a single stud bump5. The adjustment of the LC component by a single stud bump 5 simplifiesthe manufacturing process and reduces the manufacturing cost, while theadjustment of the LC component by a plurality of the stud bumps 5increases the adjusting range of the LC component.

Since the electrode pads 4 posses the L component by themselves, the LCcomponent can be adjusted by changing the shape and size of the wiringpattern of the electrode pads 4 without changing the positions of thestud bumps 5. Also, the adjustment range of the LC component adjustableby the positioning of the stud bumps 5 can be changed by changing theshape and size of the wiring pattern of the electrode pads 4. Inaddition, since there is parasitic capacitance component between theelectrode pads 4 and the IDTs 2 and also there is parasitic capacitancecomponent between the wiring patterns if the wiring patterns areinsulated therebetween like the electrode pads 4 a, the C component canbe adjusted by changing the shape and size of the electrode pads 4.

Second Embodiment

(2-1) Construction

FIG. 2 is a top view of the SAW chip 100 according to the secondembodiment of the present invention. The same reference numerals aregiven to such elements as having the similar construction as in thefirst embodiment and the explanation thereof is omitted. A wiringpattern of an electrode pad 41 according to the second embodiment issnaked. The electrode pad 41 is composed of the first to fourthbelt-like wiring patterns 41 a-41 d. The four wiring patterns 41 a-41 dare provided in substantially parallel to each other and the adjacentwiring patterns are connected to each other alternately at opposed endsthereof. The distance between the adjacent wiring patterns is madesmaller than a diameter of the stud bumps 5. Connection portions of aplurality of wiring patterns of the electrode pad 41 are enlarged sothat the stud bumps 5 can be provided at the connection portions. Theentire part of the stud bump 5 a is disposed in the connection portionof the electrode pad 41 and the stud bumps 5 b-5 h are disposed suchthat they are overlapped with two adjacent wiring patterns to cause ashort-circuit therebetween. The number of the stud bumps 5 can beincreased or decreased. Also, the stud bumps 5 can be disposed atarbitrary positions such that they are overlapped with the connectionportions between the wiring patterns or two adjacent wiring patterns.

The width of the wiring pattern of the electrode pads 4 may be varied.For example, the width of the wiring pattern may be enlarged so that theentire part of the stud bump 5 is disposed in the wiring pattern.

(2-2) Effects

The positions and the number of the stud bumps 5 can be changed so thatthe LC component can be adjusted in the same way as in the firstembodiment. Also, the wiring pattern of the electrode pads 41 is snakedand the adjacent wiring patterns are electrically short-circuited by thestud bump 5, which produces the same effect as when the L components ofthe wiring patterns are connected in parallel. Accordingly, the Lcomponent of impedance can be easily reduced. Also, the direct resistivecomponent of the wiring pattern can be easily reduced by the samereason.

An example of a plurality of the stud bumps 5 is described in the above,however, the LC component may be adjusted by a single stud bump 5. Theadjustment of the LC component by a single stud bump 5 simplifies themanufacturing process and reduces the manufacturing cost, while theadjustment of the LC component by a plurality of the stud bumps 5expands the adjustment range of the LC component.

Since the electrode pads 41 possess the L component by themselves, theLC component can be adjusted by changing the shape and size of thewiring pattern of the electrode pads 41 without changing the positionsof the stud bumps 5. Also, the adjustment range of the LC componentadjustable by the positioning of the stud bumps 5 can be changed bychanging the shape and size of the wiring pattern of the electrode pads41. In addition, there is parasitic capacitance component between thewiring patterns disposed in substantially parallel and between thewiring patterns and the IDTs 2 so that the C component can be adjustedby changing the shape and size of the wiring patterns of the electrodepads 41.

(3) Third Embodiment

(3-1) Construction

FIG. 3 is a top view of the SAW chip 100 according to the thirdembodiment of the present invention. The same reference numerals aregiven to elements having the similar construction as in the firstembodiment and the explanation thereof is omitted. Electrode pads 42a-43 d according to the third embodiment are composed of a plurality ofisland patterns which are electrically insulating therebetween. Thedistance between the adjacent island patterns is made smaller than adiameter of the stud bump 5. That is, the stud bump 5 can be disposedsuch that it is overlapped with two adjacent island patterns. The numberof the stud bumps 5 provided in the respective electrode pads 42 a-42 cis increased or decreased. The stud bump 5 can be disposed on a singleisland pattern like the stud bump 5 a or on two adjacent patterns likethe stud bumps 5 b-5 e. The number of the stud bumps 5 provided in therespective electrode pads 42 a-42 c is determined according to the shapeand size of the respective island patterns and the number of the islandpatterns.

(3-2) Effects

The positions and the number of the stud bumps 5 can be changed so thatthe LC component can be adjusted in the same way as in the firstembodiment. Although it is made insulating between the respective islandpatterns of the electrode pads 42, the island patterns are put in thesame condition as if they are connected to each other from the beginningby connecting them by the stud bumps 5. Accordingly, the degree offreedom of the electrically connected patterns according to the thirdembodiment is broader than that in the second embodiment, wherein thewiring patterns are connected to each other from the beginning, thusincreasing the adjustment range of the LC component. Also, since it ismade insulating between the island patterns, it is easier in the thirdembodiment than in the first and second embodiments, wherein the wiringpatterns are connected to each other from the beginning, to provide openstubs without any restriction.

Since the island patterns possess the L component by themselves, the Lcomponent and direct resistive component can be adjusted by changing theshape and size of the island pattern without changing the positions ofthe stud bumps 5. Also, the adjustment range of the LC componentadjustable by the positioning of the stud bumps 5 can be enlarged bychanging the shape and size of the island pattern. In addition, there isparasitic capacitance component between the island patterns and betweenthe island patterns and the IDTs 2 so that the C component can beadjusted by changing the shape and size of the island patterns.

According to the present invention, in a surface acoustic wave device,electrode patterns capable of electrical connection with each other at aplurality of positions are provided and stud bumps are disposed suchthat a desirable LC component of the surface acoustic wave device atelectrode pads is obtained so that it is easy to adjust the LCcomponent.

1. A surface acoustic wave device comprising: a piezoelectric substrate; at least one inter-digital transducers (IDT) provided on said piezoelectric substrate; at least one electrode pad electrically connected to said IDT; and at least one stud bump disposed on said electrode pad such that an LC component of said surface acoustic wave device has a predetermined value.
 2. The surface acoustic wave device according to claim 1, wherein said electrode pad is elongated.
 3. The surface acoustic wave device according to claim 1, wherein said electrode pad has a first wiring pattern and a second wiring pattern connected to each other at an end thereof and said stud bump is disposed on said electrode pad such that said first and second wiring patterns are electrically short-circuited.
 4. The surface acoustic wave device according to claim 1, wherein said electrode pad has a plurality of wiring patterns, adjacent wiring patterns connected to each other at an end thereof and said stud bump is disposed on said electrode pad such that said adjacent wiring patterns are electrically short-circuited.
 5. The surface acoustic wave device according to claim 1, wherein said electrode pad has a first island pattern and a second island pattern and said stud bump is disposed on said electrode pad such that said first and second island patterns are electrically short-circuited.
 6. The surface acoustic wave device according to claim 1, which further comprises a base substrate having a wiring pattern corresponding to said electrode pad, wherein said stud bump is electrically connected to said wiring pattern of said base substrate.
 7. A method of adjusting an LC component of a surface acoustic wave device having at least one stud bump and at least one electrode pad, said method comprising the steps of: determining a position at which said stud bump is disposed on said electrode pad such that said LC component of said surface acoustic wave device has a predetermined value; and disposing said stud bump at said determined position on said electrode pad.
 8. The method according to claim 7, wherein said electrode pad is elongated.
 9. The method according to claim 7, wherein said electrode pad has a first wiring pattern and a second wiring pattern connected to each other at an end thereof and said first and second wiring patterns are electrically short-circuited by said stud bump disposed at determined position on said electrode pad.
 10. The method according to claim 7, wherein said electrode pad has a plurality of wiring patterns, adjacent wiring patterns being connected to each other at an end thereof and said adjacent wiring patterns are electrically short-circuited by said stud bump disposed at determined position on said electrode pad.
 11. The method according to claim 7, wherein said electrode pad has a first island pattern and a second island pattern and said first and second island patterns are electrically short-circuited by said stud bump disposed at said determined position on said electrode pad.
 12. The method according to claim 7, which further comprises, after said step of disposing said stud bump on said electrode pad, a step of mounting said surface acoustic wave device onto a base substrate having a wiring pattern corresponding to said electrode pad. 